This application claims the priority of Japanese Patent Applications No. 2001-87392 filed on Mar. 26, 2001 which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a television standard converter for endoscope in which a video signal of NTSC system is converted into a video signal of PAL system.
2. Description of the Related Art
In Japan, an electronic endoscopic apparatus has a monitor of an NTSC (National Television System Committee) system, but it is demanded that the electronic endoscopic apparatus may have a monitor of a PAL (Phase Alternation by Line) system that is employed mainly in Europe. The number of scanning lines (horizontal lines) for this television monitor is 525 in the NTSC system, or 625 in the PAL system. Through the interpolation process of a video signal conversion circuit, the data of 525 scanning lines is converted into the data of 625 scanning lines.
That is, a CCD (Charge Coupled Device) as a solid image pickup element adapted to the scanning lines of the NTSC system is provided at a top end of an electronic endoscope, whereby an output signal of this CCD undergoes a predetermined video processing to acquire a signal (horizontal line data of 525 lines), which is once stored at a timing of the NTSC system in a memory, and read at a timing of the PAL system from this memory. And this signal is converted into the horizontal line data of 625 lines through an interpolation process to increase the number of scanning lines, and form a video signal adapted to the PAL system.
However, in the signal conversion between the NTSC system and the PAL system in the electronic endoscopic apparatus, there was a problem that a bar noise occurs horizontally on the monitor, because they have a different vertical scanning period. Namely, the vertical synchronization signals of both television standards are shown in FIGS. 5A and 5B, in which a field signal is obtained for a vertical scanning period of 1/60 sec. (about 16.7 ms) in the NTSC system as shown in FIG. 5A, while a field signal is obtained for a vertical scanning period of 1/50 sec. (20 ms) in the PAL system as shown in FIG. 5B.
FIG. 6 shows a state in which a write operation of video signal data overtakes a read operation in an NTSC-PAL conversion memory. If a write period of the odd field signal data D51 (51st) of the NTSC system overlaps a read period of the odd field signal data D50 (50th) of the PAL system, the writing of the next data overtakes the reading of the data in the PAL system at the time of line Ln, for example. Then, the bar noise occurs horizontally at the time of overtaking, and a video of one field is formed in the video data at different time, whereby if there is a movement in the observed subject, an unclear video is displayed on the monitor.